In recent years, a hybrid vehicle comprising an engine and a motor generator is provided for the purpose of saving a fuel for driving the engine, and reducing a noise resulting from a driving of the engine and emission resulting from combustion of a fuel. This kind of hybrid vehicle is driven by controlling the engine and the motor generator according to a running condition of the vehicle.
Specifically, the hybrid vehicle can be run by driving the engine in a revolution region where combustion efficiency thereof is preferable, and also by using the motor generator as a motor while halting the engine in a revolution region where the combustion efficiency of the engine is degraded. One example of a control system for a drive system thus having an engine and a motor generator is disclosed in Japanese Patent Laid-Open No. 2003-127681. According to the system disclosed in Japanese Patent Laid-Open No. 2003-127681, an output torque of an internal combustion engine is transmitted to a wheel via a planetary gear mechanism, a transmission, a propeller shaft, a differential device and a vehicle axle. The planetary gear mechanism comprises a sun gear, ring gear and a carrier, and a crankshaft of the engine is connected with the carrier. Also, a first motor/generator is provided and a rotor thereof is connected with the sun gear. The ring gear is connected with the transmission. On the other hand, a second motor/generator is provided and a rotor thereof is connected with the propeller shaft. Japanese Patent Laid-Open No. 2003-127681 describes that torque of the vehicle axle is generated by the internal combustion engine and the second motor/generator, and a transmission having a plurality of planetary gear mechanisms, a clutch and a brake is disclosed therein as an example of the transmission. A first gear position, a second gear position, a third gear position and a reverse position of the transmission are achieved by controlling an engagement and a releasing of the clutch and the brake.
According to the vehicle disclosed in Japanese Patent Laid-Open No. 2003-127681, if an upshift operation is carried out to reduce the gear ratio of the transmission when an accelerator is on, in other words, when the torque of the internal combustion engine is being transmitted the wheel via the planetary gear mechanism and the transmission, the output torque of the transmission may be changed as a result of engaging and releasing operations of the clutch and the brake. Consequently, a shocks may be amplified.